The present invention relates to a star coupler used in an optical Local Area Network (LAN) or the like, and particularly to a star coupler which can be interconnected.
There have been proposed star couplers interconnectable in use (for example, refer to Ota: "Interconnectable Star Couplers", proceeding of 1991 Spring Meeting of The Institute of Electronics, Information and Communication Engineers of Japan, B910 (1991)). There has been further proposed an optical communication network constituted by interconnectable star couplers connected through an optical amplifier (for example, refer to Takeshi Ota: "Coupled Star Network: A New Configuration for Optical Local Area Network", IEICE Trans. Commun. Vol. E75-B, No. 2, pp. 67-75 (1992)). There has been furthermore proposed an interconnectable star coupler having input and output terminals integrated into one (for example, refer to Ota: "Coupled Star Network", proceeding of 1992 Spring Meeting of The Institute of Electronics, Information and Communication Engineers of Japan, B1020 (1992) and Ota: "Four-port Multimode Interconnectable Star Coupler", Electronics Lett., Vol. 29, No. 10, pp. 919-920 (1993)). FIG. 7 shows an example of a conventional interconnectable star coupler having input and output terminals integrated into one.
In the conventional four-terminal interconnectable star coupler shown in FIG. 7, two optical fibers 2 are coupled on each of the two opposite end surfaces of a rectangular substrate 1, and respective terminals Ta, Tb, Tc and Td are coupled through optical waveguides 5, one-to-two asymmetric optical couplers 6, and symmetric optical couplers 7.
An optical signal S inputted from one terminal Ta of the four-terminal star coupler shown in FIG. 7 is branched into two waveguides at the ratio of 2:1 through the asymmetric optical coupler 6. The optical signal passed through one of the two waveguides (on the smaller branching ratio side) is outputted as it is to the opposite terminal Tb. On the other hand, the optical signal passed through the other waveguide (on the larger branching ratio side) is further branched through the symmetric optical coupler 7 and outputted to the other two terminals Tc and Td. Since the respective terminals Ta, Tb, Tc and Td of the star coupler in FIG. 7 are symmetrical, an optical signal inputted into any one terminal is not distributed to the one terminal but is uniformly distributed to terminals other than the one terminal. That is, an interconnectable star coupler is realized.
In the four-terminal star coupler shown in FIG. 7, however, light passes two curved lines (utilizing circular arcs here) that sandwich a straight line therebetween in the optical coupler 7, so that the light is apt to leak in the portion where the light is incident from the straight line to the curved lines to increase a data loss (refer to FIGS. 8(a) and 8(b)). Further, such a loss also is produced in the optical coupler 6.
Generally, light propagated in an optical waveguide leaks in accordance with the degrees of change of the curvature of the line when the curvature changes. Particularly in the case where a linear waveguide 9 is connected to a curved waveguide 8 as shown in FIG. 8(a), two bendings are produced so that leakage is caused in each of the two bendings. Other than this, if two curved lines 8 and 8 different in polarity of curvature, that is, having positive and negative curvatures respectively, are continuously connected as shown in FIG. 8(b) (two bendings are produced also in this case), leakage is produced in the portion where light is incident from one curved line 8 to the other curved line 8.
This fact corresponds to the fact that the difference between the number of modes propagatable in a linear optical waveguide and the number of modes propagatable in a curved optical waveguide becomes a loss in the case of a multi-mode optical waveguide. Light can be guided in a mode propagatable in a curved optical waveguide as long as the curvature of the optical waveguide does not change. Accordingly, such a loss depends on the change of curvature and the number of times of change, and does not depend on the length of the optical waveguide as long as the curvature does not change.
Also in such a conventional six-terminal star coupler as shown in FIG. 9, losses are produced in bent portions of optical couplers 6 and 7. The example shown in FIG. 9 has a structure such that in a certain path, a terminal is connected to another terminal through not less than two bendings, so that the losses caused by the bendings becomes larger.